Parking lock for an automatic transmission and method for operating the parking lock

ABSTRACT

In a parking lock for an automatic transmission, an electrical parking lock actuator has an electric motor, a spur gear stage driveable by the electric motor, and a worm gear stage driveable by the spur gear stage. A worm shaft of the worm gear stage is connected rotationally conjointly to an output gear of the spur gear stage, and a worm gear of the worm gear stage is connected rotationally conjointly to a transmission-side parking lock shaft. The worm shaft is fixed in an axially displaceable manner in the output gear. A stop limits the pivoting movement of the worm gear when a parking lock position of the parking lock shaft is reached. A spring braces the worm shaft in an axial direction of the worm shaft against a holding mechanism which is situated in a stop position, in which the holding mechanism is fixed by an electrically energized electromagnet.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to German Patent Application No. DE 102017 103 317.3, filed Feb. 17, 2017, which is incorporated by referenceherein in its entirety.

FIELD OF THE INVENTION

The invention relates to a parking lock for an automatic transmission,and to a method for operating a parking lock of said type.

BACKGROUND OF THE INVENTION

The majority of automatic transmissions of motor vehicles available onthe market have a parking lock which is mechanically actuated by meansof a cable pull connected to a selector lever. New developments provide,in some cases, an electrohydraulic or electromotive actuation means,which is actuated electrically by the selector lever. This is known asfull shift by wire function. If a vehicle is equipped with a fullyautomatic parking function, in which the driver may even be situatedoutside the vehicle during the parking process, the vehicle mustautomatically engage the parking lock. This is only possible with atransmission with full shift by wire function.

To avoid new development, or in the case of low equipment rates of thefull shift by wire function, and in the case of an approach stronglygeared towards using identical parts to form the base transmission, useis made of electromotive parking lock actuators situated outside thetransmission. Actuator shift modules are known which are bolted onto theoutside of the transmission and which are connected directly to aparking lock shaft of the transmission, and actuator shift modules areknown which are fastened to the transmission or to the vehicle and whichactuate the parking lock lever via a cable pull.

The vehicle must be capable of being securely parked, on all roadwaygradients, even in the event of a failure of the on-board electricalsystem. Vehicles with an electric parking brake and an electric parkinglock actuator therefore require either a second redundant electricalsupply or a device which automatically engages the parking lock in thissituation.

A parking lock for an automatic transmission, wherein the parking lockis actuable by means of an electric parking lock actuator, wherein theparking lock actuator has an electric motor, a spur gear stage which isdriveable by means of the electric motor, and a worm gear stage which isdriveable by means of the spur gear stage, wherein a worm shaft of theworm gear stage is connected rotationally conjointly to an output gearof the spur gear stage, and a worm gear of the worm gear stage isconnected rotationally conjointly to a transmission-side parking lockshaft, is known from practice. Said electric parking lock actuator isbolted onto the transmission and is used in a purely electricallyoperated passenger motor vehicle (BMW i3). Said parking lock actuator isnot equipped with a fail-safe function, because a redundant electricalsupply is already available in electric vehicles.

EP 1 228 328 B1, which is incorporated by reference herein, discloses atransmission actuator arrangement with an electromagnetic clutch which,in the event of an electrical failure, is decoupled only by means of aspring element, and thus the shift position “P” of the transmission andthe parking lock is engaged.

SUMMARY OF THE INVENTION

Described herein is a parking lock known from practice such that theparking lock function is reliably activated in the event of anelectrical failure, in particular a failure of the on-board electricalsystem of the vehicle. This should be ensured in the case of a compactdesign of the parking lock. Also described is art advantageous methodfor operating a parking lock said type.

In the case of the parking lock according to aspects of the invention,the worm shaft is fixed in an axially displaceable manner in the outputgear. A stop limits the pivoting movement of the worm gear when aparking lock position of the parking lock shaft is reached. A springbraces the worm shaft in an axial direction of the worm shaft against aholding means which is situated in a stop position, in which the holdingmeans is fixed by means of an electrically energized electromagnet. Whenthe parking lock shaft is situated in a non-parking lock position, andwhen the holding means is not held in the initial position by theelectromagnet as a result of the latter being electrically deenergized,the worm shaft is displaced in an axial direction under the action ofthe spring and pivots the worm gear into its stop position.

Thus, if the electrical supply of the vehicle, that is to say theon-board electrical system, fails in the switching position of thenon-parking lock position, the electromagnet is no longer electricallyenergized. The spring moves the worm shaft axially against the stop, inparticular a housing stop, and thus rotates the worm gear into theswitching position of the parking lock position, without the worm shaft,the spur gear stage and the electric motor rotating in the process. Theparking lock is thus automatically engaged in the electricallydeenergized state of the vehicle. In the case of such a parking lockactuator, a redundant electrical supply is not required for thisoperating situation.

The worm gear is in particular positioned such that, in the non-parkinglock position, it is positioned so as to face toward the spur gearstage, and in the parking lock position, it is positioned so as to facetoward the spring. Here, the spring acts via the worm shaft on the wormgear such that, in the event of an electrical failure, the worm shaft,and thus also the worm gear are moved away from the spur gear stage.

The holding means is preferably formed as a holding lever. The latter ismounted so as to be pivotable about a static axle. One lever arm of theholding lever can be placed into an operative position with theelectromagnet, and the other lever arm of the holding lever can beplaced in operative connection with the worm shaft. When theelectromagnet is electrically energized, it holds the holding lever,whereby the holding lever does not pivot but rather remains in saidposition and thus forms the counterbearing for the axially displaceableworm shaft. In the electrically deenergized state of the electromagnet,this function of the counterbearing is eliminated in that the holdinglever pivots away from the electromagnet under the action of the spring,and thus, likewise under the action of the spring, the pivot shaft isdisplaced axially, with the worm gear which meshes with the worm shaftsimultaneously being pivoted, with the parking lock shaft connected tothe worm gear being transferred into its parking lock position.

In a preferred refinement of the invention, it is provided that the wormshaft receives, axially fixed therein, a holding bracket which makescontact with the holding means situated in the stop position. By meansof said holding bracket, the forces can be transmitted in an optimummanner between the latter and the holding means, specifically theholding lever.

The holding means is preferably preloaded under the action of a furtherspring against the electromagnet. In particular, one lever arm ispreloaded against the electromagnet. If the electromagnet iselectrically deenergized and is pivoted under the action of theworm-gear-side spring, said further spring has the effect, when such apivoting-out force no longer acts on the, holding means, that theholding means is pivoted back against the electromagnet again.

In the method according to aspects of the invention for operating theparking lock, it is provided that, during the disengagement of theparking lock, the electric motor rotates the worm shaft, which issupported via the holding means on the electromagnet, in a forwarddirection of rotation when the electromagnet is electrically energized.The statement “in a forward direction of rotation” is to be understoodto mean that the worm shaft is rotated in a first axial direction. Bycontrast, during the engagement of the parking lock, the electric motorrotates the worm shaft, which is supported on the output gear of thespur gear stage, in a backward direction of rotation, and therefore inthe axial direction opposite to the direction of rotation during thedisengagement. In the case of an electrical failure and thus anelectrically deenergized electromagnet and interrupted action on theholding means, and when the parking lock is disengaged, the worm shaftis displaced axially under the action of the spring, with the worm gearsimultaneously being pivoted in order to engage the parking lock.

In accordance with the force conditions to be implemented betweenholding means and electromagnet in the respective method steps, saidelectromagnet is preferably electrically energized to different extents.Accordingly, during the disengagement of the parking lock, it isprovided that the electromagnet is electrically energized to a maximumextent. When the parking lock is disengaged, it is provided that theelectric motor is electrically energized to a reduced extent. During theengagement of the parking lock, it is provided that the electromagnet iselectrically energized to a reduced extent.

Following a failure of the on-board electrical system and an axialdisplacement of the worm shaft in order to attain the parking lockposition, it is provided in particular that a reset function beimplemented in the event of a renewed supply of electricity. Here, bymeans of the electric motor, the worm shaft is rotated backward, suchthat the worm shaft moves in the direction of the output gear into itsinitial position. By means of the further spring, the holding means ismoved against the electromagnet, and the electromagnet is electricallyenergized particular electrically energized to a reduced extent.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the invention will emerge from the claims, from theappended drawing and from the description of the preferred exemplaryembodiment depicted in the drawing, without the invention beingrestricted thereto.

In the drawing:

FIG. 1 shows, in a schematic illustration, a parking lock for anautomatic transmission, which has a parking lock actuator according tothe prior art known from practice,

FIG. 2 shows, in a schematic illustration, a parking lock for anautomatic, transmission, which has a parking lock actuator according toaspects of the invention,

FIG. 3 shows a detail of the parking lock actuator shown in FIG. 2,

FIGS. 4-10 show different operating states of the parking lock actuatoraccording to aspects of the invention shown in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

The parking lock according to the prior art shown in FIG. 1 is used inan automatic transmission. The parking lock is actuable by means of anelectric parking lock actuator 1. Said parking lock actuator has anelectric motor 2, a spur gear stage 3 which is driveable by means of theelectric motor 2, and a worm gear stage 4 which is driveable by means ofthe spur gear stage 3. The spur gear stage 3 has a pinion 6 connected toan output shaft 5 and has an output gear—spur gear 7—which meshes withthe pinion 6. The spur gear 7 is axially fixed in a bearing 8 and isfixedly connected to a worm shaft 9 of the spur gear stage 4. In theregion of the end averted from the bearing 8, the worm shaft 9 ismounted in a further bearing 10 which is fixed with respect to ahousing. The worm shaft 9 meshes with a worm gear 11 of the worm gearstage 4, which extends over a quarter circle. The worm gear 11 isconnected rotationally conjointly to a parking lock shaft 12 of theautomatic transmission.

The described parts of the parking lock actuator 1 are arranged within ahousing 13. Within this, the worm gear 11 and thus the parking lockshaft 12 fixedly connected thereto are pivotable between the illustratedparking lock position P and a non-parking lock position “non-P”,wherein, to attain the latter position, the worm gear 11 is pivotedthrough an angle of approximately 25° counterclockwise from the position“P”. Said pivoting is performed exclusively under the action of theelectric motor 2 in one or the other direction of rotation of its rotor.

With regard to the parking lock actuator 1 according to aspects of theinvention as per the illustration in FIG. 2, reference is firstly made,in order to avoid repetitions, to the description of the actuator as perthe prior art in FIG. 1; differences will emerge from the followingdescription: The parking lock actuator 1 according to aspects of theinvention differs from the parking lock actuator 1 known from the priorart as per FIG. 1 in that the worm shaft 9 is mounted in, an axiallydisplaceable manner in the spur gear 7 by means is of a driver toothing14 between spur gear 7 and worm shaft 9. The worm shaft 9 is thuscoupled rotationally conjointly, and displaceably in the direction ofthe axis of rotation, to the spur gear 7 of the spur gear stage 3. It isfurthermore provided that the worm shaft 9 is acted on by spring force,specifically the force of a spring 15 which is illustrated in FIG. 2 andwhich is in the form of a helical spring, in an axial direction awayfrom the spur gear stage 3. The spring 15 is supported on the housing 13and, in the “non-P” switching position, is held in a prestressed stateby an electromagnet 16 and by a mechanical device to be described inmore detail below. In the event of a failure of the electrical supply ofthe electromagnet 16 in the “non-P” switching position, the worm shaft 9is displaced axially by the spring 15 and rotates the worm gear 11, andthe parking lock shaft 12 connected rotationally conjointly thereto,into the “P” switching position. As a result of backward rotation of theworm shaft 9 from the emergency switching position, the normal positionof the parking lock actuator 1 is attained again.

During normal operation, switching between the switching positions “P”and “non-P” (“R” in most gearboxes) is performed by virtue of theelectric motor 2 rotating the parking lock shaft 12 via the spur gearstage 3 and the worm gear stage 4. The overall transmission ratiobetween the electric motor 2 and the parking lock shaft 12 amounts to115, for example. The worm shaft 9 and the spur gear 7 are connected toone another rotationally conjointly, and so as to be displaceablerelative to one another in the direction of the axis of rotation, bymeans of the driver toothing 14. The spur gear 7 is mounted rotatably ina bearing 8 which is fixed with respect to a housing. The worm shaft 9is supported, by means of a shaft shoulder 17, axially in one directionon the spur gear 7 and in the other direction on a holding bracket 18which engages with a U-shaped profile 19 into a groove 20 of the wormshaft 9. The holding bracket 18 is held in its position, and isprevented from rotating, by means of a holding lever 22 which is mountedin the housing 13 so as to be rotatable about an axis 21. The holdinglever 22 is held in its position by an electromagnet 16 which is fixedin the housing 13. By means of a lever ratio at the holding lever 22,which has a long, electromagnet-side lever arm 23 and a short,holding-bracket-side lever arm 24, axial forces that act on the wormshaft 9 as reaction forces from the worm toothing only have to be heldin reduced form by the electromagnet 16.

The spring 15 axially fixed in the bearing 10 acts permanently on theholding bracket 18 and, during fault-free, operation, remainspermanently inactive in said position. The axial forces of the spring 15are likewise supported by means of the electromagnet 16. When thevehicle is parked and the electromagnet 16 is electrically deenergized,the worm gear 11 is situated in the switching position P and bearsagainst a housing stop 25. This prevents axial forces of the spring 15from axially moving the worm shaft 9. The self-locking action of thehigh transmission ratio also prevents axial forces of the spring 15 fromleading to an undesired rotation of the electric motor 2. A furtherspring 26 which is arranged on the rotary axle 21 of the holding lever22 and which is in the form of a leg spring ensures that the holdinglever 22 bears in a defined manner against the electromagnet 16 even inthe electrically deenergized state.

If the on-board electrical system fails in the switching position“non-P”, the electromagnet 16 is no longer electrically energized. Thespring 15 moves the worm shaft 9 axially as far as the housing stop 25,and rotates the worm gear 11 into the switching position “P”, withoutthe worm shaft 9, the spur gear stage 3 and the electric motor 2rotating in the process. The parking lock is thus automatically engagedin the electrically deenergized state of the vehicle. In the case ofthis parking lock actuator 1, a redundant electrical supply is thus notrequired for this operating situation.

In the exemplary embodiment according to aspects of the invention, theworm gear 11 has a stop 25 in the housing 13 in the “P” position. Thespring 15 transmits the force via a disk 27 and the U-shaped holdingbracket 18 to the worm shaft 9. The holding bracket 18 transmits theforce in a groove 20 of the worm shaft 9. The holding bracket 18 extendsin a longitudinal direction over the spring 15 at least over a length ofthe actuation stroke of the spring 15. The holding lever 22 is mountedrotatably in the housing 13 and, in the normal position, prevents theactuation stroke. The holding lever 22 is held in the normal position bythe electromagnet 16. In particular, the holding lever 22 is composed ofplastic with a clipped-in steel disk 28 for the purposes of interactingwith the electromagnet 16.

The electrically actuable parking lock actuator 1 is bolted as aswitching module to the outside of the transmission. The electric motor2 rotates the parking lock shaft 12, which projects out of thetransmission, with a high self-locking overall transmission ratio bymeans of the spur gear stage 3 and the worm gear stage 4. A motorcontrol unit is preferably integrated in the actuator housing.

FIGS. 4 to 10 show the individual operating states and switchingpositions and illustrate the functioning of the parking lock actuator 1according to aspects of the invention.

FIG. 4 shows the operating state in the case of a parked vehicle andswitched-off ignition. Here, the parking lock is engaged, and theelectromagnet 15 is electrically deenergized. The spring 15 is held inposition by the worm shaft 9, which is in a self-locking state. The wormshaft 9 is held axially by the worm gear 11. The worm gear 11 bearsagainst the stop 25.

FIG. 5 shows the operating state of the switched-on ignition. Here, theelectromagnet 16 is electrically energized. Since neither switchingforces nor spring forces act on the holding lever 22 in this switchingposition, it is possible, with corresponding actuation, for electricalenergization to be performed with reduced power. The worm shaft 9 isheld axially by the worm gear 11. The worm gear 11 bears against thestop 25.

FIG. 6 shows the operating state of the disengagement of the parkinglock. The electric motor 2 is energized, and the worm shaft 2 rotatesthe worm gear 11 into the switching position “non-P”. For this purpose,the electromagnet 16 is energized with full power. The spring force ofthe spring 15 and the reaction forces on the worm toothing are supportedon the electromagnet 16 via the holding lever 22. When the electricmotor 2 is actuated, the worm shaft 9 rotates in a forward direction ofrotation. The pivoting of the worm gear 11 is performed from the “P”position into the “non-P” position through an angle of approximately25°.

FIG. 7 shows the operating state of the disengaged parking lock. Here,the worm gear 9 is situated in the switching position “non-P”, in whichit has been rotated through approximately 25°, and is not in contactwith the rear housing stop. The electromagnet 16 only has to impart aholding force that counteracts the spring force of the spring 15. Toreduce the power losses, actuation with reduced power is possible.

FIG. 8 shows the operating state of the engagement of the parking lock.Here, the electric motor 2 is actuated in the backward direction ofrotation, and rotates the worm gear 11 into the switching position “P”.The worm shaft 9 is supported, by means of its shaft shoulder 17,axially on the spur gear 7. The electromagnet 16 likewise supports onlythe switching spring force. Here, too, actuation of the electromagnet 16is possible with reduced power.

FIG. 9 shows the operating state of a failure of the on-board electricalsystem. If the on-board electrical system fails when the parking lock isdisengaged, the electromagnet 16 is no longer electrically energized.The spring 15 is not subjected to any holding force by the holding lever22, and, via the holding bracket 18, moves the worm shaft 9 in an axialdirection such that the worm gear 11 rotates as far as the housing stop25 of the switching position “P”. No rotation of worm shaft 9, spur gearstage 3 and electric motor 2 occur here. The parking lock isautomatically engaged in the electrically deenergized state of thevehicle. The holding lever 22 is moved into a tilted position owing tothe displaced position of the holding bracket 18, and is held in saidposition owing to the design of the holding bracket 18, which extendsover the spring 15.

Such a mechanical fail-safe switching position is detected by means ofposition sensors on the worm shaft 9. If, after said switching process,an energy source provides a supply to the on-board electrical systemagain, the actuation of the electric motor 2 initially performs a resetoperation, which is illustrated in FIG. 10, in order to restore theinitial state of the parking lock actuator 1. For this purpose, theelectric motor 2 rotates the worm shaft 9 backward until the worm shaft9 reassumes its initial position for abutment against the spur gear 7.The spring 15 is stressed again. Here, the worm gear 11 remains in theswitching position “P”. The holding lever 22 is, by means of the furtherspring 26, rotated with the steel disk 28 for abutment against theelectromagnet 16. The initial state of the parking lock actuator 1 isthus restored with full functionality.

LIST OF REFERENCE DESIGNATIONS

1 Parking lock, actuator

2 Electric motor

3 Spur gear stage

4 Worm gear stage

5 Output shaft

6 Pinion

7 Spur gear

8 Bearing

9 Worm shaft

10 Bearing

11 Worm gear

12 Parking lock shaft

13 Housing

14 Driver toothing

15 Spring

16 Electromagnet

17 Shaft shoulder

18 Holding bracket

19 U-shaped profile

20 Groove

21 Axle

22 Holding lever

23 Lever arm

24 Lever arm

25 Housing stop

26 Spring

27 Disk

28 Steel disk

What is claimed is:
 1. A parking lock for an automatic transmission, theparking lock being actuable by an electric parking lock actuator havingan electric motor, a spur gear stage being driveable by the electricmotor, a worm gear stage being driveable by the spur gear stage, a wormshaft of the worm gear stage is connected rotationally conjointly to anoutput gear of the spur gear stage, and a worm gear of the worm gearstage is connected rotationally conjointly to a transmission-sideparking lock shaft, the worm shaft is coupled in an axially displaceablemanner in the output gear, a stop limits the pivoting movement of theworm gear when a parking lock position of the parking lock shaft isreached, a spring braces the worm shaft in an axial direction of theworm shaft against a holding means which is situated in a holding meansstop position, in which the holding means is fixed by an electricallyenergized electromagnet, when the parking lock shaft is situated in anon-parking lock position, and when the holding means is not held in theholding means stop position by the electromagnet as a result of theelectromagnet being electrically deenergized, the worm shaft isconfigured to be displaced in an axial direction under the action of thespring to pivot the worm gear into a worm gear stop position.
 2. Theparking lock as claimed in claim 1, wherein the spur gear stage has apinion and the output gear, the pinion being connected to an outputshaft of the electric motor, wherein the output gear of the spur gearstage has a driver toothing for the worm shaft.
 3. The parking lock asclaimed in claim 1, wherein the worm shaft is configured to hold thespring in position, as a result of a self-locking action, when theelectromagnet is electrically deenergized.
 4. The parking lock asclaimed in claim 1, wherein the worm gear extends over a part of eithera circle or a quarter circle.
 5. The parking lock as claimed in claim 1,wherein, in the non-parking lock position, the worm gear is positionedso as to face toward the spur gear stage, and in the parking lockposition, said worm gear is positioned so as to face toward the spring.6. The parking lock as claimed in claim 1, wherein the holding means isa holding lever which is mounted so as to be pivotable about a staticaxle, wherein one lever arm of the holding lever can be placed into anoperative position with the electromagnet, and another lever arm of theholding lever is configured to be placed in operative connection withthe worm shaft.
 7. The parking lock as claimed in claim 6, wherein theworm shaft receives, axially fixed therein, a holding bracket whichmakes contact with the holding means situated in the holding means stopposition.
 8. The parking lock as claimed in claim 7, wherein the springis supported with one end on a positionally fixed stop and with anotherend on the holding bracket or on a disk which bears against the holdingbracket.
 9. The parking lock as claimed in claim 6, wherein the holdingmeans is at least partially of magnetic form at the electromagnet side,and the holding means receives a steel disk in the region of said onelever arm.
 10. The parking lock as claimed in claim 6, wherein theholding means is preloaded under the action of a further spring againstthe electromagnet, and said one lever arm s preloaded against theelectromagnet.